Someone recently asked me to explain the differences between the 10-foot tap rule and the 25-foot tap rule as they apply to feeders. An additional question was raised about connecting multiple taps to the same feeder using both the 10- and 25-foot tap rules. Using examples helps simplify the approach to properly applying the National Electrical Code (NEC) tap rules.
Sections 240.4 and 240.21 require conductors to be protected at their rated and calculated ampacities and to protect them generally at the point where they receive their supply. This is not always possible, so the Code permits tap conductors under restrictive conditions. One of the first factors to verify is that the main feeder being tapped multiple times has the required ampacity and is properly protected in accordance with 240.4. This is accomplished by verifying the load on the single feeder, which would include all the loads connected to each of the multiple feeder taps. Next, size the conductor properly in accordance with 310.15, and apply any correction factors or adjustments for ambient temperature. Finally, protect the main feeder with a properly rated fuse or circuit breaker.
Response and comparison
The main differences between the 10- and 25-foot feeder tap rules are length and the minimum size required for the tap conductors. Sections 240.21(B)(1) and (B)(2) support this. The rest of the requirements in each of these sections are similar and deal with physical damage, terminating in a properly rated device, and sizing the tap conductors to handle the calculated load.
As for the second question, it is permitted to make multiple taps to a single feeder using both 10- and 25-foot taps, as long as the respective requirements for each tap rule are applied. The 10-foot tap rule obviously limits the tap conductor length to 10 feet, and the 25-foot tap rule limits the tap conductor length to 25 feet. The minimum sizes for the tap conductors differ. For the 10-foot rule, the tap conductor must be not less than one-tenth the rating of the overcurrent device protecting the main feeder to which the tap is being made. The 25-foot tap rule requires the tap conductors to be not less than one-third of the rating of the overcurrent device protecting the main feeder to which the tap is made. See the examples below.
In the first example, a 400-ampere (A) breaker protects a main feeder and one 10-foot feeder tap. The main feeder conductors are 600 kcmil copper, and the minimum required size conductor for the feeder tap is not less than 8 AWG copper (40A conductor), which is one-tenth of the size of the 400A main feeder breaker (see Figure 1).
In the second example, we have a 400A breaker protecting a main feeder and one 25-foot feeder tap. The size of the main feeder conductors is 600 kcmil copper, and the minimum required conductor size for the feeder tap is not less than 1/0 AWG copper (conductor ampacity of 150A), which is more than one-third of the size of the 400A main feeder breaker (see Figure 2).
In the third example, there are three 10-foot taps and one 25-foot tap to the same 400A feeder. The size of the main feeder conductors once again is 600 kcmil copper. The minimum required conductor size for the 25-foot feeder tap is not less than 1/0 AWG THW copper (150A conductor), which is more than one-third of the size of the 400A main feeder breaker. There is one 10-foot, 40A tap and two 50A, 10-foot feeder taps in addition to the 25-foot tap. The minimum size THW copper conductor required for the 10-foot taps calls for 40- and 50A conductors, respectively. Size 8 THW copper meets those requirements (see Figure 3).
These examples are kept very simple and do not account for any correction factors or ampacity adjustments for ambient temperature that may have to be applied in a real-world application.
As a reminder, these 10-foot tap conductors must have an ampacity of not less than the combined calculated loads on the circuits supplied by the tap conductors. They must also have an ampacity not less than the rating of the device supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors. The 10-foot tap conductors must have an ampacity of at least one-tenth of the 400A main feeder overcurrent device. The 25-foot tap conductors must be rated at least one-third of the size of the 400A overcurrent device protecting the main feeder. Section 240.21(B) contains all of the requirements for 10- and 25-foot feeder taps.